The Permian–Triassic extinction event, commonly known as the Great Dying, happened at the boundary between the Permian and Triassic geological periods, marking the end of the Paleozoic era and the start of the Mesozoic era. It is Earth’s most severe extinction event, causing the loss of 57% of biological families, 62% of genera, 81% of marine species, and 70% of terrestrial vertebrate species. It also caused the greatest known mass extinction of insects. This event is the most extreme of the "Big Five" mass extinctions during the Phanerozoic eon. Evidence suggests one to three distinct phases of extinction, with the largest marine extinction occurring over 60,000 to 100,000 years around 251.902 million years ago, at the Permian-Triassic boundary. There is debate about whether land extinctions happened at the same time as the main marine extinction.

Scientists agree that the main cause was massive volcanic eruptions in the Siberian Traps, which released sulfur dioxide and carbon dioxide. These emissions led to oxygen-starved, sulfurous oceans, higher global temperatures, and ocean acidification. Atmospheric carbon dioxide levels increased from about 400 ppm to 2,500 ppm, with 3,900 to 12,000 gigatonnes of carbon added to the ocean-atmosphere system. Other possible factors include carbon dioxide released from burning oil and coal deposits, methane emissions from methane ice deposits, methane produced by new microorganisms fed by minerals from eruptions, stronger and longer El Niño events, and an extraterrestrial impact that formed the Araguainha crater, releasing methane and damaging the ozone layer, which increased exposure to solar radiation.

Dating

Previously, scientists believed that rock layers from the Permian–Triassic boundary were too few and incomplete to study in detail. However, it is now possible to determine the timing of the extinction with very precise dates. Uranium–lead zircon dating from five volcanic ash layers at the Global Stratotype Section and Point for the Permian–Triassic boundary in Meishan, China, has created a detailed timeline for the extinction. This timeline helps scientists explore how global environmental changes, disruptions to the carbon cycle, mass extinction, and recovery were connected over thousands of years. The first appearance of the conodont Hindeodus parvus is used to mark the Permian–Triassic boundary.

The extinction occurred between about 251.941 and 251.880 million years ago, lasting approximately 60,000 years. A large and sudden drop in δ C, the ratio of the stable isotope carbon-13 to carbon-12, happened during this time. This drop is often used to identify the Permian–Triassic boundary and the Permian–Triassic Mass Extinction event (PTME) in rocks that cannot be dated using radiometric methods. The decrease in the carbon isotope ratio was between 4% and 7% and lasted about 500,000 years. However, measuring its exact value is difficult because many rock layers near the boundary have been altered by natural processes.

Evidence suggests that the temperature increased by about 8 degrees Celsius (14 degrees Fahrenheit) and that carbon dioxide levels rose to 2,500 parts per million (ppm). For comparison, carbon dioxide levels before the Industrial Revolution were 280 ppm, and today they are about 426 ppm. There is also evidence that ultraviolet radiation from the sun increased, causing mutations in plant spores.

Some scientists believe the Permian–Triassic boundary is linked to a sudden increase in the number of fungi in both marine and terrestrial environments. This increase, called a "fungal spike," may have been caused by the large amount of dead plants and animals that fungi fed on. Some paleontologists use this "fungal spike" to identify rock layers near the Permian–Triassic boundary in areas where radiometric dating is not possible or where there are no suitable index fossils. However, the idea of a fungal spike has been criticized. For example, Reduviasporonites, the most common spore associated with the spike, may actually be a type of algae. The spike did not appear globally, and in some places, it did not occur at the Permian–Triassic boundary. New chemical evidence supports that Reduviasporonites originated from fungi, which reduces some of these criticisms.

There is uncertainty about how long the overall extinction lasted and how long it took for different groups of organisms to go extinct. Some evidence suggests that the extinction happened in several waves or was spread out over millions of years, with a sharp peak in the last million years of the Permian. Statistical studies of rock layers in Meishan, Zhejiang Province, China, suggest that the main extinction occurred around one peak. However, studies of other locations, such as the Liangfengya and Shangsi sections, found evidence of two extinction waves with different causes. Recent research shows that different groups of organisms went extinct at different times. For example, ostracod and brachiopod extinctions were separated by about 670,000 to 1.17 million years. Analysis of Lopingian rock layers in the Bowen Basin of Queensland indicates repeated periods of marine stress before the end-Permian extinction, supporting the idea that the extinction was gradual. The decline in marine species and the collapse of marine ecosystems may have happened separately, with the decline in species richness occurring about 61,000 years before the ecosystem collapse.

Whether the extinction of land and marine life happened at the same time or at different times is still debated. Evidence from well-preserved rock layers in east Greenland suggests that land and marine extinctions began together. In these layers, the decline in animal life occurred over about 10,000 to 60,000 years, while plants showed the full impact of the event several hundred thousand years later. Many rock layers in South China show that land and marine extinctions were synchronous. Research in the Sydney Basin also supports that the extinction of land and marine life happened at the same time. However, other scientists believe that land extinctions began 60,000 to 370,000 years before marine extinctions. Chemical analysis of rock layers in Finnmark and Trøndelag shows that changes in plant life occurred before the large drop in the carbon isotope ratio during the marine extinction. Dating of rock layers in the Karoo Basin indicates that land extinctions happened before marine extinctions. The Sunjiagou Formation in South China also records a decline in land ecosystems before the marine crisis. Other studies show that in tropical regions, land extinctions occurred after marine extinctions.

Studying the timing and causes of the Permian–Triassic extinction is complicated by the Capitanian extinction, also called the Guadalupian extinction, which occurred just before the Permian–Triassic event. It is unclear whether the Capitanian extinction had already ended by the time the Permian–Triassic event began. Some extinctions once thought to have happened at the Permian–Triassic boundary have been re-dated

Extinction patterns

Marine invertebrates experienced the most severe losses during the P–Tr extinction, although earlier estimates of 90–96% marine species extinction were based on confusion with the end-Capitanian mass extinction, which occurred 7–10 million years earlier. Evidence of these losses was found in samples from south China near the P–Tr boundary. In these samples, 286 out of 329 marine invertebrate genera disappeared within the final two sedimentary layers containing Permian conodonts. The drop in diversity was likely due to a sudden rise in extinctions, not a decrease in new species forming.

The extinction mainly affected organisms with calcium carbonate skeletons, especially those that relied on stable CO₂ levels to build them. These organisms were vulnerable to ocean acidification caused by increased atmospheric CO₂. Organisms using hemocyanin or hemoglobin for oxygen transport were more resistant to extinction than those using hemerythrin or oxygen diffusion. Evidence shows that being endemic (found only in a specific area) increased a taxon’s risk of extinction. Bivalve species limited to a single region were more likely to go extinct than those found worldwide. Survival rates varied little across latitudes. Organisms in areas less affected by global warming experienced fewer or delayed extinctions.

Among benthic organisms, the extinction event increased background extinction rates, causing the greatest loss for taxa with naturally high extinction rates (and high turnover). The extinction rate for marine organisms was extremely severe. Bioturbators (organisms that mix sediment) were heavily impacted, as shown by the loss of the mixed sediment layer in many marine environments during the end-Permian extinction.

Surviving marine invertebrates included articulate brachiopods (those with a hinge), which had declined slowly since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which nearly went extinct but later became abundant. Groups with high survival rates generally had active circulation control, complex gas exchange systems, and light calcification. More heavily calcified organisms with simpler breathing systems suffered the most species loss. Among brachiopods, surviving species were typically small and rare compared to earlier, more diverse communities.

Conodonts experienced severe declines in both species and body shape diversity, though not as badly as during the Capitanian mass extinction.

Ammonoids, which had been declining for 30 million years since the Roadian (middle Permian), faced a major extinction event 10 million years before the main P–Tr event, at the end of the Capitanian stage. This early extinction greatly reduced the variety of ecological roles these organisms filled. Diversity and variety fell further until the P–Tr boundary, where the extinction was not selective, suggesting a catastrophic cause. During the Triassic, diversity rose quickly, but variety remained low. The range of possible shapes and structures among ammonoids became more limited as the Permian progressed. A few million years into the Triassic, the original range of ammonoid forms was reoccupied, but the distribution of these traits changed among groups.

Ostracods faced long-term diversity changes during the Changhsingian period before the main P–Tr extinction, when many vanished suddenly. At least 74% of ostracods died during the P–Tr extinction itself.

Bryozoans had been declining throughout the Late Permian before suffering even greater losses during the P–Tr extinction, becoming the most severely affected lophophorate group.

Deep-water sponges lost much diversity and had smaller spicules during the P–Tr extinction. Shallow-water sponges were less affected, showing larger spicules and less loss of body shape diversity.

Foraminifera faced a severe drop in diversity. Evidence from South China shows their extinction had two phases. Foraminiferal biodiversity hotspots shifted to deeper waters during the P–Tr extinction. Approximately 93% of late Permian foraminifera became extinct, including 50% of the Textulariina clade, 92% of Lagenida, 96% of Fusulinida, and 100% of Miliolida. Foraminifera with calcium carbonate shells had a 91% extinction rate. Lagenides survived more than fusulinoidean fusulinides, possibly because they had greater environmental tolerance and wider geographic ranges.

Cladodontomorph sharks may have survived the extinction by living in deep ocean refuges, as suggested by the discovery of Early Cretaceous cladodontomorphs in deep, outer shelf environments. Ichthyosaurs, which evolved just before the P–Tr extinction, also survived.

The Lilliput effect, where species shrink during and after mass extinctions, was observed near the Permian-Triassic boundary in foraminifera, brachiopods, bivalves, and ostracods. While surviving gastropods were smaller than those that died, it is unclear if the Lilliput effect applied to gastropods. Some gastropod groups, called "Gulliver gastropods," grew larger after the extinction, showing the opposite trend, known as the Brobdingnag effect.

The Permian had high insect and invertebrate diversity, including the largest insects ever. The end-Permian extinction was the largest known mass extinction for insects, with eight to nine orders becoming extinct and ten others losing diversity. Palaeodictyopteroids (insects with piercing mouthparts) declined during the mid-Permian, possibly due to changes in plant life. The greatest decline occurred in the Late Permian, likely not directly caused by weather-related plant changes. Some insect declines during the P–Tr extinction were due to biogeographic shifts rather than outright extinctions.

The geological record of terrestrial plants is limited, relying mostly

Biotic recovery

After the extinction event, the way ecosystems were organized changed based on the types of organisms that survived. In the ocean, the "Paleozoic evolutionary fauna" decreased in numbers, while the "modern evolutionary fauna" became more common. The Permian-Triassic mass extinction was a major turning point in this change, which began after the Capitanian mass extinction and continued until the Late Jurassic. Typical types of bottom-dwelling marine life included bivalves, snails, sea urchins, and Malacostraca, while bony fish and marine reptiles became more common in open ocean areas. On land, dinosaurs and mammals appeared during the Triassic period. The change in the types of organisms that existed was partly because the extinction event affected some groups more than others, such as brachiopods being more severely impacted than bivalves. Recovery also varied among groups. Some species that survived eventually went extinct without recovering, while others became dominant over time, such as bivalves.

Immediately after the end-Permian extinction, a time of widespread species distribution began. Marine life after the extinction had few species and was dominated by a few types of organisms, such as bivalves like Claraia, Unionites, Eumorphotis, and Promyalina, conodonts like Clarkina and Hindeodus, brachiopods like Lingularia, and foraminifera like Earlandia and Rectocornuspira kalhori (sometimes classified as Ammodiscus). The variety of different types of organisms was also low. Marine life after the Permian-Triassic mass extinction had little difference in species diversity across different latitudes.

Scientists disagree about how quickly life recovered after the extinction. Some say it took 10 million years until the Middle Triassic because the extinction was so severe. However, studies in Bear Lake County, Idaho, and nearby areas showed a faster recovery in a specific marine ecosystem during the Early Triassic, with life rebounding in about 1.3 million years. In Italy, a complex group of trace fossils appeared less than a million years after the extinction, and in China, the Guiyang biota and Shanggan fauna show evidence of life thriving just a million years after the event. Other regions, such as those with the Wangmo biota in Guizhou and a gastropod group in Oman, also show signs of recovery. Differences in recovery speed suggest that the extinction’s effects were not the same everywhere, with some areas recovering faster due to less environmental stress. High-latitude ecosystems may have recovered faster because of increased productivity after the extinction. Some studies suggest that while species diversity recovered quickly, the variety of ecological roles took much longer to return to pre-extinction levels. One study found that marine ecosystems were still recovering 50 million years after the extinction, during the latest Triassic, even though species diversity had recovered in a tenth of that time.

Recovery speed and timing also depended on the type of organism and their lifestyle. Seafloor communities remained less diverse until the end of the Early Triassic, about 4 million years after the extinction. Organisms living on the seafloor took longer to recover than those living in the sediment. This slow recovery contrasts with the quick recovery of swimming organisms like ammonoids, which reached pre-extinction diversity levels two million years after the extinction, and conodonts, which diversified rapidly in the Early Triassic.

Recent research suggests that recovery speed was influenced by competition between species, which affects how quickly new species form. Slow recovery in the Early Triassic may have been due to low competition because of few species, while faster recovery in the Anisian period may have been caused by increased competition as niches filled. This created a cycle where more biodiversity led to more competition, which in turn led to more biodiversity. Other explanations suggest that recovery was delayed because harsh conditions returned repeatedly during the Early Triassic, causing further extinctions, such as the Smithian-Spathian boundary event. Extremely hot temperatures and high carbon dioxide levels may have harmed vulnerable species, like those with skeletons. The slow recovery of bottom-dwelling organisms may have been due to low oxygen levels, but the presence of many benthic species contradicts this. A 2019 study found that differences in recovery times between ecosystems were due to varying environmental stress after the extinction, with areas facing more stress recovering more slowly. These repeated environmental challenges limited the complexity of marine ecosystems until the Spathian. Recovery ecosystems remained uneven and unstable into the Anisian, making them vulnerable to further stress.

Most marine communities fully recovered by the Middle Triassic, but global marine diversity reached pre-extinction levels no earlier than the Middle Jurassic, about 75 million years after the extinction.

Before the extinction, about two-thirds of marine animals were attached to the seafloor. During the Mesozoic, only about half of marine animals were attached, with the rest being free-moving. Fossil evidence shows a decrease in the number of sessile, filter-feeding organisms like brachiopods and sea lilies and an increase in more complex, mobile species like snails, sea urchins, and crabs. Before the Permian extinction, both simple and complex marine ecosystems were equally common. After recovery, complex ecosystems outnumbered simple ones by nearly three to one, and increased predation and the ability to crush shells led to the Mesozoic Marine Revolution.

Marine vertebrates recovered quickly, showing complex predator-prey relationships, with vertebrates at the top of the food chain, as shown by coprolites from 5 million years after the extinction. Post-extinction hybodont sharks had extremely fast tooth replacement. Ichthyopterygians grew rapidly in size after the extinction.

Bivalves quickly returned to many marine environments after the extinction. Before the Permian-Triassic extinction, bivalves were rare but became numerous and diverse in the Triassic, taking over roles previously held by brachiopods. Bivalves were once thought to have outcompeted brachiopods, but this idea is now debated.

Hypotheses about cause

Explaining an event that happened 250 million years ago is very difficult. Much of the evidence on land has been worn away or buried deeply, and the seafloor spreads and is recycled over 200 million years, leaving little useful information beneath the ocean.

Scientists have still found a lot of evidence about the causes of the event, and several possible explanations have been suggested. These explanations include both sudden, large events and slow, gradual changes (similar to those thought to cause the Cretaceous–Paleogene extinction event, but with less agreement among scientists today).

• The sudden events include one or more large impacts from space objects, increased volcanic activity, and a sudden release of methane from the seafloor. This methane could have come from the breakdown of methane hydrate deposits or from the activity of certain microbes that produce methane.
• The slow changes include changes in sea level, increased oxygen shortages in the ocean, and increased dryness on land.

Any idea about the cause of the event must explain why it affected certain organisms more than others. It especially harmed organisms that had skeletons made of calcium carbonate. It also must explain why it took 4 to 6 million years before life started to recover and why, once recovery began, very little new biological material was formed, even though inorganic carbonates were deposited.

The flood basalt eruptions that formed the large igneous province of the Siberian Traps were among the largest volcanic events in Earth's history. These eruptions covered more than 2,000,000 square kilometers (770,000 square miles), about the size of Saudi Arabia. These eruptions happened at about the same time as the extinction event. Studies of the Norilsk and Maymecha-Kotuy regions in northern Siberia show that the volcanic activity happened in a few large bursts of magma, not in a steady flow.

The Siberian Traps caused one of the fastest increases in atmospheric carbon dioxide levels in Earth's history. The rate of carbon dioxide emissions was estimated to be five times faster than during the Capitanian mass extinction, which happened during the eruption of the Emeishan Traps. One estimate suggests that carbon dioxide levels increased from between 500 and 4,000 parts per million before the extinction to about 8,000 parts per million after. Another study estimated that carbon dioxide levels were about 400 parts per million before the extinction and then rose to 2,500 parts per million. This would have added between 3,900 and 12,000 gigatonnes of carbon to the ocean and atmosphere. These high levels of carbon dioxide would have caused extreme temperature increases. Some evidence suggests that there was a delay of 12,000 to 128,000 years between the increase in volcanic carbon dioxide emissions and the global warming. This delay could also be due to errors in the dating of the events. Before the extinction, the average global surface temperature was about 18.2 degrees Celsius, but it rose to as high as 35 degrees Celsius. This extremely hot condition lasted for up to 500,000 years. In the high southern latitudes of Gondwana, air temperatures increased by about 10 to 14 degrees Celsius. In South China, oxygen isotope shifts in conodont apatite suggest that surface water temperatures in low latitude areas increased by about 8 degrees Celsius. In present-day Iran, tropical sea surface temperatures were between 27 and 33 degrees Celsius during the Changhsingian period but jumped to over 35 degrees Celsius during the Permian-Triassic Mass Extinction Event (PTME). The higher average temperatures also led to stronger El Niño events, increasing short-term climate changes.

These very high levels of carbon dioxide lasted for a long time. The position of the supercontinent Pangaea at the time made the inorganic carbon cycle very inefficient at removing carbon from the atmosphere. In a 2020 study, scientists used a biogeochemical model to show how the greenhouse effect affected the marine environment and concluded that the mass extinction was linked to volcanic carbon dioxide emissions. Evidence also suggests that volcanic activity burned underground fossil fuel deposits, as shown by paired coronene-mercury spikes that matched widespread mercury anomalies and the rise in isotopically light carbon. Te/Th values increased twentyfold during the PTME, indicating that extreme volcanism occurred at the same time. A major influx of isotopically light zinc from the Siberian Traps also supports the idea that volcanism and the PTME happened together.

The eruptions of the Siberian Traps had unusual features that made them even more dangerous. The Siberian lithosphere contains a lot of halogens, which are very harmful to the ozone layer. Evidence from xenoliths in the subcontinental lithosphere suggests that up to 70% of the halogen content was released into the atmosphere. Around 18 teratonnes of hydrochloric acid were emitted, along with sulfur-rich gases that caused dust clouds and acid aerosols. These would have blocked sunlight and disrupted photosynthesis on land and in the ocean, causing food chains to collapse. These sulfur emissions also caused brief but severe global cooling, leading to a drop in sea levels. However, these cold events were too short to be a major cause of the extinction.

The eruptions may also have caused acid rain as the aerosols washed out of the atmosphere. This could have killed land plants and organisms with calcium carbonate shells, such as mollusks and plankton. Flood basalts usually produce fluid, low-viscosity lava that does not throw debris into the atmosphere. However, about 20% of the material from the Siberian Traps eruptions was pyroclastic ash that reached high into the atmosphere, increasing the short-term cooling effect. Once this ash had washed out of the atmosphere, the excess carbon dioxide would have remained, leading to unchecked global warming.

The burning of hydrocarbon deposits may have worsened the extinction. The Siberian Traps are located above thick layers of carbonate and evaporite deposits from the Early-Mid Paleozoic era, as well as coal-bearing rocks from the Carboniferous-Permian era. When heated by igneous intrusions, these rocks may release large amounts of greenhouse and toxic gases. The unique location of the Siberian Traps over these deposits is likely the reason for the severity of the extinction. Basalt lava erupted or intruded into carbonate rocks and sediments that formed large coal beds, emitting large amounts of carbon dioxide and leading to stronger global warming after the dust and aerosols settled. The change in the eruptions from flood basalt to sill-dominated emplacement, which released even more trapped hydrocarbon deposits, coincides with the main start of the extinction and is linked to a major negative δC excursion. The intermediate temperature of the Siberian Traps magmas helped release a large amount of carbon dioxide by heating evaporites and carbonates.

The release of methane from coal was accompanied by the explosive burning of coal and the discharge of coal fly ash. A 2011 study led by Stephen E. Grasby found evidence that volcanic activity caused massive coal beds to ignite, possibly releasing more than 3 trillion tons of carbon. They found ash deposits in deep rock layers near what is now the Buchanan Lake Formation: "coal ash dispersed by the explosive Siberian Trap eruption would be expected to have an associated release of toxic elements in impacted water bodies where fly ash slurries developed. … Mafic megascale eruptions are long-lived events that would allow significant build-up of global ash clouds." Grasby said, "In addition to these volcanoes causing fires through coal, the ash it spewed was highly toxic and was released in the land and water, potentially contributing to the worst extinction event in Earth history." However, some researchers suggest that these supposed fly ashes were actually the result of wildfires.

Comparison to present global warming

The PTME has been compared to today's human-caused global warming and the Holocene extinction because all three involve the fast release of carbon dioxide. Although the current rate of greenhouse gas emissions is much higher than the rate during the PTME, the timing and pattern of carbon release during the PTME are not fully understood. Scientists believe that carbon was likely released in short, intense bursts rather than continuously over the entire extinction period. The rate of carbon release during these bursts may have been similar to the rate of modern human-caused emissions.

Today’s oceans are also experiencing drops in pH and oxygen levels, similar to what happened during the PTME. This has led scientists to compare current environmental conditions with those of the PTME. If carbon dioxide levels continue to rise, another event similar to the biocalcification crisis observed in the fossil record may occur, which could harm modern marine ecosystems. Changes in how plants and insects interact during the PTME have also been studied as possible signs of future ecological changes.

Geologists have warned that the similarities between the PTME and today’s situation highlight the urgent need to reduce carbon dioxide emissions to avoid a similar extinction event.

Just as during the PTME, today’s oceans are undergoing major changes, including falling pH and oxygen levels. This connection is emphasized by geologist Lee Kump:

If carbon dioxide levels keep rising, it could lead to another biocalcification crisis, as seen in the fossil record. This would have serious consequences for marine life today.